The Camera to Print & Screen videos demonstrate the benefits of ETTR very clearly. I have also read Michael's earlier articles on this and I understand the reason why this works. The videos also point out the obvious fact that lower ISO settings also improve S/N ratios. Now, by exposing to the right, I am also using a slower shutter speed. If instead of exposing to the right I used the same slower shutter speed but at a lower ISO setting, I would also benefit from lower noise. My question is this: which result would give me the better image - the ETTR image at the higher speed, or the "middle"exposure at a lower ISO, using the same shutter speed and f-stop?

If instead of exposing to the right I used the same slower shutter speed but at a lower ISO setting, I would also benefit from lower noise. My question is this: which result would give me the better image - the ETTR image at the higher speed, or the "middle"exposure at a lower ISO, using the same shutter speed and f-stop?

If you use the same shutter speed and f-stop, noise will be more visible the lower the ISO. If that was not clear enough: ISO100 will display more visible noise than ISO1600.On some cameras (e.g. Pentax K5, Nikon D7000) the difference will be negligible, in some others (e.g. Canons) will be monstruous.

Canon 350D, 2 shots with the same aperture and shutter speed:

PS: if you have a digital camera, or you can borrow one from someone, it will take you 5min to take two shots with the same aperture/shutter but different ISO, develop the RAW files so that exposures match, and compare visible noise.

If the slower ISO is "pushed" during processing I can well understand this. I'll try to illustrate my original question with a concrete example. Lets say the camera's lightmeter indicated a "standard" exposure of 1/250th at f8 using ISO400. The ETTR shot might then be 1/125th at f8, also at ISO400. In ACR/Lightroom you would then back off the ETTR exposure to give a processed image with lower noise than the "standard" shot. However, how would this processed ETTR image compare to an alternative "standard" shot of 1/125th at f8, but at ISO200?

If the slower ISO is "pushed" during processing I can well understand this. I'll try to illustrate my original question with a concrete example. Lets say the camera's lightmeter indicated a "standard" exposure of 1/250th at f8 using ISO400. The ETTR shot might then be 1/125th at f8, also at ISO400. In ACR/Lightroom you would then back off the ETTR exposure to give a processed image with lower noise than the "standard" shot. However, how would this processed ETTR image compare to an alternative "standard" shot of 1/125th at f8, but at ISO200?

The 1/125th at f8 ISO400 will have LESS noise than the 1/125th at f8 ISO200.

Your "Lower ISO = more visible noise" answer is incomplete, it should have been followed by "assuming identical exposure parameters, which effectively means underexposing the lower ISO shot much more, compared to a normal exposure for that ISO."

Your "Lower ISO = more visible noise" answer is incomplete, it should have been followed by "assuming identical exposure parameters, which effectively means underexposing the lower ISO shot much more, compared to a normal exposure for that ISO."

Cheers,Bart

So are you interpreting the OP the same as I am, which seems to be the theory that rather than slow down the shutter or open the aperture to move the data to the right to achieve EttR, you would instead increase ISO which would move the data to the right in the histogram? I'm reading the question as " if the shutter speed is as slow as you want to go, and the aperture as wide as you want to go, and your exposure looks pretty good but isn't "to the right" can using ISO to move it the right help, hurt, or be irrelevant."

Logic says that if the original exposure is a good one (not grossly underexposing) increasing ISO will add noise that may not be offset sufficiently by moving the data up in the histogram.

Guillermo's example seems to show otherwise, but perhaps missing the point a little since in this case both exposures will provide an adequate exposure, the higher ISO one EttR, the other one "normal". This means only a about a .6 to 1.3 stop ISO increase, not a 4 stop one like the example, where the ISO 100 one seems to be very underexposed (I haven't seen that much noise in shadows at ISO 100 in a camera in a long time).

And Guillermo's right - curious enough to try it now. I've never thought of increasing ISO to move the data to the right because as I mentioned, logically seems counter to the concept. I'll have to do some tests while out shooting tomorrow.

Some cameras have high read noise. In these cases increasing ISO actually improves signal over read noise as the initial increase in ISO is achieved by pre amplification of sensor signal before raw conversion.

The enclosed figures shows this quite clearly. Nikon and Phase drop linearly with increasing ISO while Canon maintains it's low DR perhaps up 500 ISO before it starts to drop.

So are you interpreting the OP the same as I am, which seems to be the theory that rather than slow down the shutter or open the aperture to move the data to the right to achieve EttR, you would instead increase ISO which would move the data to the right in the histogram? I'm reading the question as " if the shutter speed is as slow as you want to go, and the aperture as wide as you want to go, and your exposure looks pretty good but isn't "to the right" can using ISO to move it the right help, hurt, or be irrelevant."

Logic says that if the original exposure is a good one (not grossly underexposing) increasing ISO will add noise that may not be offset sufficiently by moving the data up in the histogram.

Guillermo's example seems to show otherwise, but perhaps missing the point a little since in this case both exposures will provide an adequate exposure, the higher ISO one EttR, the other one "normal". This means only a about a .6 to 1.3 stop ISO increase, not a 4 stop one like the example, where the ISO 100 one seems to be very underexposed (I haven't seen that much noise in shadows at ISO 100 in a camera in a long time).

And Guillermo's right - curious enough to try it now. I've never thought of increasing ISO to move the data to the right because as I mentioned, logically seems counter to the concept. I'll have to do some tests while out shooting tomorrow.

So are you interpreting the OP the same as I am, which seems to be the theory that rather than slow down the shutter or open the aperture to move the data to the right to achieve EttR, you would instead increase ISO which would move the data to the right in the histogram? I'm reading the question as " if the shutter speed is as slow as you want to go, and the aperture as wide as you want to go, and your exposure looks pretty good but isn't "to the right" can using ISO to move it the right help, hurt, or be irrelevant."

As Guillermo has demonstrated, when we use the same aperture and shutterspeed (because we are somehow restricted to those settings), it can be advantageous to increase ISO because it can reduce the noise (although with a few caveats).

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Logic says that if the original exposure is a good one (not grossly underexposing) increasing ISO will add noise that may not be offset sufficiently by moving the data up in the histogram.

That's correct for a 'good' exposure. Guillermo's example on the other hand compares a grossly underexposed (for ISO 100) image with a much better exposure (for ISO 1600). That scenario is fine, and in sync with the OP's dilemma of a fixed aperture and shutterspeed. But is is misleading to conclude that therefore higher ISO always leads to lower noise. It does not, when the image is exposed properly for the selected ISO setting, but that would require the freedom to increase the exposure for the lower ISO setting and thus improve the photon statistics.

Therefore one needs to consider 2 scenarios:

1. Set the camera to ISO 1600, exposure meter a uniformly lit surface, and set the aperture and shutterspeed as suggested by the metering with the camera in Manual mode. Take an image. The histogram will show a hump in the middle of the range. Now, while using these same aperture and shutterspeed settings in Manual mode, change to ISO 100. Take an image. That second image will show a hump/spike on the left side of the histogram, because we now underexposed it by 4 EV for the ISO. Unsurprisingly, the earlier better exposed (for ISO 1600) image will usually exhibit lower noise, which is a useful thing to know in case of shutterspeed or aperture restrictions.

2. Now repeat the experiment, but leave the camera on (preferably) Aperture priority mode. take one shot at ISO 1600, and one shot at ISO 100. In that case the spike/hump on both histograms is roughly in the same position (the exposure levels were compatible for the given ISO setting, but with a 16x (4 EV) absolute difference in exposure time. In this case the lower ISO image will usually exhibit lower noise, because of the much improved photon statistics. In this scenario the lower ISO shot has lower noise overall.

To complicate matters, different cameras exhibit different noise characteristics depending on the chosen ISO setting. Therefore, some cameras are better not pushed beyond ISO 800 (e.g. the Canon EOS 1Ds Mark III) even for scenario 2. It can be better to underexpose (as in scenario 1.) at ISO 800 when e.g. a minimum shutterspeed is needed for a given aperture, and push in postprocessing. But that only applies to ISO 800+, otherwise there is usually nothing that beats the noise performance of more photons.

If the slower ISO is "pushed" during processing I can well understand this. I'll try to illustrate my original question with a concrete example. Lets say the camera's lightmeter indicated a "standard" exposure of 1/250th at f8 using ISO400. The ETTR shot might then be 1/125th at f8, also at ISO400. In ACR/Lightroom you would then back off the ETTR exposure to give a processed image with lower noise than the "standard" shot. However, how would this processed ETTR image compare to an alternative "standard" shot of 1/125th at f8, but at ISO200?

There is no short answer; it depends on the camera. What camera do you use?

The main point of ETTR is that increasing exposure decreases image noise. And it is important to note that ISO setting is not part of exposure -- exposure is the light intensity times exposure time of the capture, the total light captured, and so depends only on shutter speed and aperture. ISO is an amplification of the electronic signal after capture, and therefore has nothing to do with the amount of light that was captured. There are two main sources of image noise: (1) photon noise, statistical fluctuations in the number of photons in the light arriving at the sensor, and (2) electronic read noise, noise in the camera electronics which processes the signal during and after capture. The larger the exposure, the more light there is, and as a result the photon noise fluctuations are smaller relative to the amount of light captured; this is the main point of ETTR -- that noise relative to signal is smaller as exposure is increased. So if the ISO is fixed, expose to the right to improve the image noise characteristics.

Now, how does changing the ISO affect things? As far as photon noise is concerned, not at all, if exposure is kept fixed and ISO is varied; photon noise depends only on exposure, and ISO is not part of exposure. But the read noise is a property of the camera electronics, that depends on the camera design details. The second component of noise, the read noise, is ISO dependent on many cameras. Since the exposure is about light, which is made up of photons, it is useful to measure the read noise in photon equivalent units. Since the photons are converted into electrons during the capture process in the pixels, the unit of measure 'electron equivalents' is conventionally used. Here are the read noise characteristics in 'photo-electrons' (e-) as a function of ISO of a variety of cameras:

The first point is ISO 100, and each successive point is one stop higher ISO. So, on the 5D2 and D3s, read noise drops steadily until about ISO 1600 or 3200 (there is no true ISO 100 on the D3s, rather it uses ISO 200 metered for ISO 100, which is why the ISO 100 and 200 read noises are the same on this camera). So for these cameras (and all Canons, and Nikons with CMOS sensors designed by Nikon) the read noise is lowered by raising the ISO, up to about ISO 1600 or so, provided the exposure is kept fixed. Please note that this is always a secondary consideration to simply increasing the exposure, which will always lower the noise more than keeping the exposure fixed and raising the ISO. Only if you are exposure limited should you consider ETTR via increasing the ISO for cameras such as the D3/D3s/D700 or any Canon.

A new breed of cameras such as the D7000 use the Sony 'Exmor' CMOS sensor family. These turn out to have an essentially flat read noise characteristic. This means that there is no advantage to raising the ISO to reduce noise. Similarly for any camera with a CCD sensor, such as older generations of Nikon such as the D2x, or any MFDB such as the P65+ (I am ignoring here the pixel binning features of the more recent Phase offerings, which lower image read noise at the cost of reduced resolution). For all these cameras, ISO is essentially irrelevant; the only thing that controls image noise is the exposure. In fact, there is little reason to do anything other than peg the camera at or near base ISO, and set the largest exposure you can subject to the constraints of motion blur and needed DOF. Raising the ISO will do nothing but chop off highlight headroom. The meter will say you are underexposing in many cases, but a properly designed raw converter should be able to amplify the raw data accurately in the same manner as raising the camera ISO setting (note however that Adobe changes internal settings on its raw conversion based on the ISO indicated in metadata, so one will have to play around a bit to undo that; see also a parallel thread on DPP). The difference is that one can put a shoulder on the tone curve that rolls off highlights more delicately. Canon and Nikon already offer a limited version of this with Highlight Tone Priority/Active De-Lighting

Similarly for any camera with a CCD sensor, such as older generations of Nikon such as the D2x, or any MFDB such as the P65+ (I am ignoring here the pixel binning features of the more recent Phase offerings, which lower image read noise at the cost of reduced resolution). For all these cameras, ISO is essentially irrelevant; the only thing that controls image noise is the exposure. In fact, there is little reason to do anything other than peg the camera at or near base ISO, and set the largest exposure you can subject to the constraints of motion blur and needed DOF. Raising the ISO will do nothing but chop off highlight headroom.

OK, your blowing my mind here ... you're saying with my IQ180 if I shoot a shot at 1/10th at f/11 at ISO 200, and I shoot the same shot by just going to ISO 50 I won't really see any difference in the final results, even though the second one is way "under exposed"?

OK, your blowing my mind here ... you're saying with my IQ180 if I shoot a shot at 1/10th at f/11 at ISO 200, and I shoot the same shot by just going to ISO 50 I won't really see any difference in the final results, even though the second one is way "under exposed"?

That's right. There is a "but" in there however - there are side effects to be aware of - some cameras try to reduce visible noise by raising black points, many raw converters apply adjustments before you can "dial out" underexposure or ETTR, etc, so there might be some small differences - tone curve, hue shifts, etc. But side effects from the raw converter and read noise effects, etc in some cameras aside, all that counts is the number of photons that hit the sensor, aka shutter speed and aperture.

I would have suggested you try one of the converters I know applies exposure compensation correctly and doesn't jigger the controls secretly according to the ISO set in metadata (two that come to mind are RPP and RawTherapee), but unfortunately they use dcraw code to open images, and the IQ180 is not listed among the supported cameras. It's possible that the raw format is similar enough to another supported Phase One back that it will open, but no guarantees.

This is exactly what Guillermo has seen on the Pentax K5. You are actually in position to try out on IQ180.

Now, it is quite possible that a raw converter looks at ISO and applies different processing. That is a reasonable approach. Raw conversion is about actual photography and not about "objective testing".

It's a little bit parallel to some users using vendor software for superior image quality, while Jeff Schewe says that if you know how to drive LR/ACR you can get the very same results.

OK, your blowing my mind here ... you're saying with my IQ180 if I shoot a shot at 1/10th at f/11 at ISO 200, and I shoot the same shot by just going to ISO 50 I won't really see any difference in the final results, even though the second one is way "under exposed"?

Now, it is quite possible that a raw converter looks at ISO and applies different processing. That is a reasonable approach. Raw conversion is about actual photography and not about "objective testing".

I think the relevant question is why any raw converter would look only at the ISO in metadata in order to set its default processing 'look'. The ISO (International Standards Org) specification of ISO refers to output density, not to the amount of signal amplification at some intermediate stage of processing. If a raw converter is going to change its internals according to the actual ISO (which I agree can be a reasonable approach), it should use the metadata ISO together with the setting on the exposure compensation control of the converter in order to set those defaults, since that is what sets the ballpark output density (apart from sigmoidal tone curves etc). Using only the metadata makes it harder for the user to get good results using more advanced shooting techniques that take into account the camera's noise characteristics to optimize the capture.

My suggestion would be that it would be reasonable to increase default noise reduction for mages that are shot at high ISO. That could also explain differences in appearance between low and high ISO shoots.

In the other hand, it could also use the histogram data to decide optimal noise reduction. Both approaches would deceive any user who is not peeking into internal workings of converter to believe that setting higher ISO has real benefits.

Please note that I don't know if this is the case, I just suggest it could explain differences between observations and theory.

I think the relevant question is why any raw converter would look only at the ISO in metadata in order to set its default processing 'look'. The ISO (International Standards Org) specification of ISO refers to output density, not to the amount of signal amplification at some intermediate stage of processing. If a raw converter is going to change its internals according to the actual ISO (which I agree can be a reasonable approach), it should use the metadata ISO together with the setting on the exposure compensation control of the converter in order to set those defaults, since that is what sets the ballpark output density (apart from sigmoidal tone curves etc). Using only the metadata makes it harder for the user to get good results using more advanced shooting techniques that take into account the camera's noise characteristics to optimize the capture.

Wow! My head is spinning; or at least it was spinning when I first came across this thread which, at that time, ended with Reply #8. Then ejmartin came along and restored my faith in what I thought I had previously (and painfully) learned about noise and ettr.

My initial perplexity was percipitated by Mr. Luijk's terse (for him) response and his subsequent failure to elaborate upon it. I should state that I have been a fan of Guillermo's "internet escapades" for several years now, and have always found his contributions to be very high in nutritional value, though not always within the capability of my digestive system (my shortcoming, not his). Thus my trust in GL's knowledge caused me to question my own.

Now that I have had the opportunity to read all of this thread to date, it becomes clear that my confusion (and perhaps that of others) stems from the rather vague (or at least incomplete) manner in which the OP posed his question, and which was made even more perplexing (at least to me) by his addendum in Reply #2.

With apologies to Mr. Brooks, allow me to rephrase the original question and supply my answers, so that others my confirm or critique my understanding of the problem:

Q1. My in-camera meter suggests an exposure of S sec at aperture A, with my chosen ISO setting. I know from experience that this reading allows at least 1.5 - 2.0 stops of headroom before there is any danger of serious highlight clipping. I wish to expose to the right in order to minimize noise in the image. In order to ettr (push the histogram to the right) I have the option to increase the exposure (by altering either S or A) or, alternatively, to increase the ISO setting. Which of these alternatives will produce an image with the least noise?

A1. Increasing the exposure by changing either shutter speed or aperture will produce the cleaner image. Increasing the ISO will add noise to the image.

Q2. I have set my shutter speed and aperture to values dictated by the shot and which I prefer not to alter. I have set the ISO to a value such that the in-camera meter shows a "proper" exposure. My only alternative to achieve ettr (push the histogram to the right) is to increase the ISO. Will doing so improve, degrade or have no effect on the noise in the image?

A2. If you are using a camera whose read noise diminishes with increasing ISO then increasing ISO up to 1600 will lessen noise. If you are not using such a camera, there is nothing to be gained by increasing ISO. In either case, be aware that increasing the ISO comes at a cost of decreasing highlight headroom and may cause some clipping.

And finally the question which (I believe) was never asked, but nevertheless answered by GL:

Q3. I know that increasing the ISO of my camera, especially above X, results in very noisy images. However the exposure parameters I have chosen require an ISO of X or more to produce a properly exposed image. Can I reduce the noise in the image by shooting at an ISO less than X and compensating for the underexposure in post-processing?

Q1. My in-camera meter suggests an exposure of S sec at aperture A, with my chosen ISO setting. I know from experience that this reading allows at least 1.5 - 2.0 stops of headroom before there is any danger of serious highlight clipping. I wish to expose to the right in order to minimize noise in the image. In order to ettr (push the histogram to the right) I have the option to increase the exposure (by altering either S or A) or, alternatively, to increase the ISO setting. Which of these alternatives will produce an image with the least noise?

A1. Increasing the exposure by changing either shutter speed or aperture will produce the cleaner image. Increasing the ISO will add noise to the image.

The first part -- increasing the exposure results in a cleaner image -- is correct. Increasing the ISO for fixed exposure will not add noise to the image, as may be seen in any of the examples I gave, increasing ISO at worst does not change the noise, and in many examples results in less noise.

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Q2. I have set my shutter speed and aperture to values dictated by the shot and which I prefer not to alter. I have set the ISO to a value such that the in-camera meter shows a "proper" exposure. My only alternative to achieve ettr (push the histogram to the right) is to increase the ISO. Will doing so improve, degrade or have no effect on the noise in the image?

A2. If you are using a camera whose read noise diminishes with increasing ISO then increasing ISO up to 1600 will lessen noise. If you are not using such a camera, there is nothing to be gained by increasing ISO. In either case, be aware that increasing the ISO comes at a cost of decreasing highlight headroom and may cause some clipping.

Do you see that this answer is in contradiction with the your first one?

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Q3. I know that increasing the ISO of my camera, especially above X, results in very noisy images. However the exposure parameters I have chosen require an ISO of X or more to produce a properly exposed image. Can I reduce the noise in the image by shooting at an ISO less than X and compensating for the underexposure in post-processing?

A3. See Reply #1

The answer is no, you cannot reduce the noise through use of a lower in-camera ISO. The noise at fixed exposure will at best be about the same. The advantage of a lower ISO, if present, is not in the arena of noise but rather in added highlight headroom.

The first part -- increasing the exposure results in a cleaner image -- is correct. Increasing the ISO for fixed exposure will not add noise to the image, as may be seen in any of the examples I gave, increasing ISO at worst does not change the noise, and in many examples results in less noise.

Yes, that is clear. I meant simply to emphasize that, if one wishes to push the histogram to the right, whenever possible increasing exposure will always produce the best result.

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Do you see that this answer is in contradiction with the your first one?

Yes. In A1 I recognised that increased exposure reduces noise but jumped to the conclusion that increasing ISO adds to noise; I knew better.

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The answer is no, you cannot reduce the noise through use of a lower in-camera ISO. The noise at fixed exposure will at best be about the same. The advantage of a lower ISO, if present, is not in the arena of noise but rather in added highlight headroom.

Yes, Guillermo's answer made that clear. Again many thanks.

P.S. Are you perhaps the same person as one Emil Martinec at the U. of Chicago (my alma mater, class of 1952). His (your?) paper on "Noise, Dynamic Range and Bit Depth" was most enlightening and a pleasure to read; even if partly over my head.

P.S. Are you perhaps the same person as one Emil Martinec at the U. of Chicago (my alma mater, class of 1952). His (your?) paper on "Noise, Dynamic Range and Bit Depth" was most enlightening and a pleasure to read; even if partly over my head.